If you think about it, you will realise that Knuth's command definition syntax:
\def\blah#1#2 ... #9{<macro body>}
is intrinsically limited to just 9 arguments. There's no direct way
round this: how would you express a 10th argument? - and ensure that
the syntax didn't gobble some other valid usage?
If you really must have more than 9 arguments, the way to go is:
\def\blah#1#2 ... #9{%
\def\ArgI{{#1}}%
\def\ArgII{{#2}}%
...
\def\ArgIX{{#9}}%
\BlahRelay
}
\def\BlahRelay#1#2#3{%
% arguments 1-9 are now in
% \ArgI-\ArgIX
% arguments 10-12 are in
% #1-#3
<macro body>%
}
This technique is easily extendible by concert pianists of the TeX
keyboard, but is really hard to recommend.
LaTeX users have the small convenience of merely giving a number of
arguments in the \newcommand that defines each part of the
relaying mechanism: Knuth's restriction applies to \newcommand
just as it does to \def. However, LaTeX users also have the
way out of such barbarous command syntax: the keyval
package. With keyval, and a bit of programming, one can
write really quite sophisticated commands, whose invocation might look
like:
\flowerinstance{species=Primula veris,
family=Primulaceae,
location=Coldham's Common,
locationtype=Common grazing land,
date=1995/04/24,
numplants=50,
soiltype=alkaline
}
The merit of such verbosity is that it is self-explanatory: the typist
doesn't have to remember that argument twelve is soiltype, and so
on: the commands may be copied from field notes quickly and
accurately.